1. Field of the Invention
The present invention relates generally to resolver systems and, more particularly, to resolver-to-digital converters (RDCs) for these systems.
2. Description of the Related Art
FIG. 1 illustrates a conventional resolver system 20 that includes a resolver 22 and an RDC 24. The resolver 22 has a rotor 26 that rotates in a stator 28 and is coupled to a rotatable member 30 whose angular position is defined by an angle xcex8. A rotor winding 32 is carried on the rotor 26 and is excited by a carrier signal 34 (typically a sinusoid having a frequency that is generally in the range of 2-20 KHz) from a carrier signal generator 36. In addition, first and second stator windings 40 and 42 are carried on the stator 28.
In response to the carrier signal 34 in the rotor winding 32, sense signals are magnetically coupled into the stator windings 40 and 42 as they rotate with the rotatable member 30. The amplitudes of the sense signals change as the relationships between the stator windings and the rotor winding change with variations in the angle xcex8. The stator windings 40 and 42 are electrically offset from each other by an offset angle. This offset angle is typically 90xc2x0 so that the sense signals are generally in quadrature.
In a quadrature relationship, the rotor windings 40 and 42 provide sense signals A1 sin xcex8 and A2 cos xcex8 to the RDC 24 wherein the sense signals are modulated onto the carrier signal 34. The RDC 24 receives the sense signals A1 sin xcex8 and A2 cos xcex8 and the carrier signal 34 and generates a digital estimate xcfx86 of the angle xcex8 of the rotatable member 30. Typically, RDCs also provide an analog estimate xcfx86 of the velocity of the rotatable member 30.
Resolver systems are used in a variety of applications (e.g., automotive motion sensing and control, automotive motor control, industrial and servo motor control, encoder emulation and factory automation) for providing an estimate xcfx86 of a position angle xcex8 of rotatable members (e.g., vehicle steering wheels, aircraft control structures, robot arms, gyroscope members, machine tools and scanned antennas) over a wide range of member velocities (e.g., 0-1 Khz).
In these applications, the resolver 22 of FIG. 1 may be located at a substantial distance from the RDC 24 and, accordingly, the sense signals A1 sin xcex8 and A2 cos xcex8 are typically coupled through transmission lines (e.g. twisted pairs) 44. However, the length, position, environment (e.g., vibration) and physical state (e.g., worn or crimped) of the cables or the physical state of the resolver (e.g., inoperative) may cause the sense amplitudes A1 and A2 to vary significantly from their nominal value Anom (or, in the extreme, cause a complete loss of sense signals). If the variation is excessive, the accuracy of the digital estimate xcfx86 may be degraded to a level at which it should not be relied on by circuits or systems that receive it.
It has been proposed, for example, that the orientation of a vehicle""s front wheels be controlled in response to a resolver system that provides an estimate xcfx86 of the angle xcex8 of the vehicle""s steering wheel. If the accuracy of the estimate xcfx86 has been degraded for any reason, it is imperative for safety that the coupling between the steering wheel position and the steering actuator be immediately replaced with a backup system.
It is desirable, therefore, that a resolver system include a reliable fault detection system that can provide meaningful fault signals Sfault (provided from the RDC 24 in FIG. 1). Although some conventional resolver systems have included fault detection systems, their performance parameters (e.g., sensitivity, accuracy and response time) have typically lacked the ability to detect out-of-specification conditions and this ability is critical in many modern applications of resolver systems.
The present invention is directed to resolver systems that generate an estimate xcfx86 of a rotatable member""s position angle xcex8 and provide fault signals which monitor the reliability and accuracy of the estimate.
Fault signals of the invention are derived from a monitor signal which multiplies resolver and estimate signals to derive information on the absolute and relative levels of resolver sense signals. At least one fault signal is derived from a loop error signal of the system servo loop. These fault signals report on, for example, mismatched sense signals, out-of-range sense signals and loss of position tracking to thereby enhance accuracy and safety in various resolver applications.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.